The development of biological control agents as alternatives to chemical insecticides for the control of important pest species is a subject of increasing interest. Concerns for the environment and exposure of man to harmful substances in air, food and water have stimulated legislation and restrictions regarding the use of chemical pesticides, particularly for pests found in the urban environment. Control of insect pests in urban areas is highly desirable but exposure to chemical pesticides in the household and from lawns and gardens is of great concern to the public. If given a choice, most people would use a non-toxic biological control rather than a toxic chemical to control insects in the urban environment. The problem is that very few biological alternatives to chemical insecticides are available for purchase and use by the public.
Bacillus thutingiensis (B.t.) produces an insect toxin designated as .delta.-endotoxin. It is synthesized by the B.t. sporulating cell. The toxin, upon being ingested in its crystalline form by susceptible insects, is transformed into biologically active moieties by the insect gut juice proteases. The primary target is insect cells of the gut epithelium, which are rapidly destroyed.
The reported activity spectrum of B.t. covers insect species within the order Lepidoptera, many of which are major pests in agriculture and forestry. The activity spectrum also includes the insect order Diptera, which includes mosquitos and black flies. See Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thutingiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C. C., (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Krieg, et al. (1983) Z. ang. Ent. 96:500-508, describe a B.t. isolate named Bacillus thuringiensis var. tenebrionis, which is reportedly active against two beetles in the order Coleoptera. These are the Colorado potato beetle, Leptinotarsa decemlineata, and Agelastica alni. In European Patent Application No. 0 202 739 there is disclosed a novel B.t. isolate active against Coleoptera. It is known as B. thuringiensis var. san diego (B.t.s.d.). U.S. Pat. No. 4,966,765 discloses the coleopteran-active Bacillus thuringiensis isolate B.t. PS86B1.
Ants comprise a large group of insects (family Formicidac) from the taxonomic order, Hymenoptera. They are among the most common house pests. In many situations, ants are a nuisance pest. Foraging ants create problems with hygiene in hospitals and the food industry. Ants also create problems in agriculture. Damage can be caused by direct feeding on plants. Harvester and fire ants are commonly associated with this type of damage (Holldobler, B., E. O. Wilson [1990] The Ants, Belkap Press, Cambridge, Mass. 732 pp.) Some ants cause indirect damage by nurturing and protecting sap feeding insects such as mealybugs and aphids. Ants, particularly in the genus Solenopsis are capable of producing extremely painful stings to humans. It has been estimated that approximately 10,000 stings occur each year (Habermehl, G. G. [1981] Venomous Animals and Their Toxins, Springer-Verlag, N.Y., 195 pp.). The pharaoh ant (Monomodum pharaonis) is primarily an urban pest. However, this species can also be an agricultural pest and damage to corn has been noted (Ebeling, W. [1978] Urban Entomology, UC Press, Berkeley, Calif., 695 pp.).
Carpenter ants, Camponotus spp., are distributed throughout North America. Some of the more common and/or studied species include C. toodoe in the Pacific northwest, C. clarithorax in southern California, and the black, red, and Florida carpenter ants, C. pennsylvanicus, C. noveboracensis and C. abdominalis, respectively, in the east (Ebeling, W. [1978] Urban Entomology, Univ. Calif.: Berkeley p. 209-213). Public concern over carpenter ants has been increasing due to the greater probability of structural infestations as suburban developments extend into the forest habitats of the ants.
Pestiferous species of carpenter ants may be considered nuisance pests because of their foraging activity inside homes. More significant damage occurs when carpenter ants extend their nests into sound wood. Nesting sites may be located in live and dead trees, sometimes resulting in damage to shade trees. Nests may also be established in walls and support beams of structures, or in voids within doors, walls, and furniture. Preference for moist or decaying wood has been reported, but nesting sites are not restricted to such areas. Carpenter ant populations develop relatively slowly with colonies of 300-2,000 workers being produced over a 2-year or longer period for various species. The presence of reproductives follows this slow development since their production has been reported only from well established colonies (Hansen, L. D., R. D. Akre [1985] Biology of carpenter ants in Washington state (Hymenoptera: Formicidae: Camponotus). Melanderia 43. 62 p.; Pricer, J. L. [1908] Biol. Bull. 14:177-218). Despite the slow colony growth, large colonies with satellite colonies have been found. Worker movement occurs between the main colony and the satellites, which serve as areas for further brood development and colony expansion (Hansen and Akre [1985], supra).
Current methods for controlling structural infestations of carpenter ants include sanitation of potential and current nest sites, minimizing access to structures (eg. preventing the contact of tree branches with a structure), and the application of insecticides to repel (perimeter spray barriers) and/or eliminate carpenter ants. The use of boric acid dust in dry, wall voids is reported to be effective for up to 20 years (Hansen and Akre, supra).
Recommendations for the chemical control of established structural infestations in the home are often accompanied with warnings of possible hazards to the applicator as well as children and pets. Alternative control methods such as effective biological control agents have not been found (Akre, R. D., L. D. Hansen, A. L. Antonelli [1989] Ext. Bull. Washington State Univ. Coop. Ext. Serv. 1989 rev. no. EB 0818, 6 pp.).
A need clearly exists for a safe, effective biological control agent for carpenter ants.
Pharaoh ants, Monomoriurn pharaonis, have been described as " . . . the most persistent and difficult of all our house-infesting ants to control or eradicate" (Smith, M. R. [1965] USDA-ARS Tech. Bull. No. 1326, 105 pp.). It is a tropical species which has extended its range to more temperate regions by establishing colonies in heated buildings. Pharaoh ants frequently infests buildings where food is prepared, and have been found to carry pathogenic organisms (Beatson, S. H. [1972] Lancet 1:425-427).
The difficulty in controlling pharaoh ants may be attributed to their inaccessible nesting sites, rapid population growth, and dispersion of colonies. Their small size allows establishment of colonies in any suitable location, including unusual places such as between books and in stored clothing. With multiple queen colonies, and the warm (30.degree. C.), humid (63-80% RH) conditions that favor pharaoh ants, large colonies can develop rapidly. Portions of these large colonies may disperse to form new colonies at any time, probably in response to overcrowding and unfavorable microenvironmental conditions. Unlike other ant species, pharaoh ants do not exhibit intercolony aggression. This permits the adoption of ants from other colonies and may further enhance the establishment of new colonies and reinfestations. Pharaoh ants also forage for food more than 35 m from the nest without distinct trail following, and thus make nests difficult to find and eradicate.
Control methods for pharaoh ants emphasize the use of insect growth regulators (IGR) or toxicants incorporated into baits. Properly implemented bait programs are effective, however it may take over a month to achieve control. Insecticide applications, while fast acting, usually do not eliminate colonies, and may be unacceptable in certain areas where toxic residues are a concern. In addition, insecticide applications are generally not compatible with bait programs.
A need exists for safe and effective biological control agents for pharaoh ants.